Cementitious boards useful in the construction industry are known to contain inorganic, hydraulically setting material, such as Portland cement or gypsum. Hydraulic gypsum and cement, once set, have very little tensile strength and are usually reinforced with facing materials which improve the resistance to tensile and flexural loads. This has been the basis for using paper facing on conventional gypsum wall board and non-woven glass fiber scrim in cement boards.
Glass fiber meshes have been popular as a facing sheet in cement boards because they can increase the dimensional stability in the presence of moisture and provide greater physical and mechanical properties. However, most glass fiber compositions, other than AR glass, degrade in the alkali environment of a cement core, so they must be coated with a protective finish.
Cementitious boards have been manufactured by casting a hydraulic cement mixture in the form of a thin, indefinitely long panel. See U.S. Pat. No. 4,504,335, which is hereby incorporated by reference. The hydraulic cement is usually a mortar containing a mixture of water and Portland cement, sand, mineral or non-mineral aggregate, fly ash, accelerators, plasticizers, foaming agents and/or other additives. The mortar slurry is deposited onto a glass reinforcing network having a strippable paper sheet thereon, which is fed from a roll to pass over a forming table and under a continuous stream of mortar. The mortar is then distributed across the breadth of the carrier sheet, and the mortar-laden carrier sheet is towed through a slit defined by a supporting surface and a cylindrical mortar screeding roller mounted above the supporting surface so that its axis is transversely parallel to the supporting surface. The long network of reinforcing fibers is drawn against the roller and through the slit, rotating the roller counter to the direction of the travel of the carrier sheet, whereby the roller presses the network into the surface of the mortar and wipes mortar adhering to the roller into the interstices of the network. The network then tows the resulting broad, flat ribbon of mortar towards a cutter.
Similarly, British Patent Specification No. 772,581 teaches a production of reinforced plasterboard by a method which comprises spreading plaster on a first conveyor belt, dumping the plaster onto a plaster-soaked reinforcing mesh which is being transported by a second conveyor belt, and passing the plaster under a pressure roller to produce a ribbon of the required thickness. A second plaster-soaked mesh is dragged onto the upper surface of the ribbon as the mesh is fed under a third conveyor belt mounted above and in pressing relationship to the ribbon of plaster.
In still another process, as described in Lehnert et al., U.S. Pat. No. 4,647,496, a randomly oriented fibrous glass mat is fed onto a continuously moving belt onto which gypsum slurry is poured. The top surface of the gypsum ribbon thus formed is layered with a second randomly oriented glass mat which forms a sandwich with the gypsum core and the lower glass mat.
Other fiber and coating technologies possibly useful in fabric reinforcements include those found in U.S. Pat. Appl. 2006/0188719 (now U.S. Pat. No. 7,045,209); U.S. Pat. Nos. 4,532,275 and 3,600,269; and Beren, J. R.; Heterophasic polypropylene copolymer resins for extrusion coating, Polymer, Laminations and Coatings Conference 1994, pp 102-112 (1994); Marques, M. V; Poloponsky, M; Chaves, G. E; Influence of the elastomeric polypropylene addition on the properties of commercial metallocenic polypropylene, Mat. Res., 4, 4, (2001); and Mascia, L; Dhillon, J; Harper, M. F; Adhesions Enhancement of rubbery and ductile polyolefin coatings on glass fibers for epoxy composites and effects on failure mechanism, Journal of Applied Polymer Science, 47, 3, pp 487-498, (1993), which are hereby incorporated herein by reference in their entirety.
Woven knit and laid scrim fabrics used in cementitious boards may be coated either:
(a) before fabric-forming, as in single-end-coated fabrics;
(b) in-line (normally roller or dip coated) concurrently with formation such as in the case of laid scrim non-woven meshes; or
(c) off-line coated after formation (normally roller or dip coated), typically used with many woven fabrics. In the case of coating before fabric-forming, the cost of coating each strand individually, in an operation prior to weaving, may be prohibitive. In the cases of in-line or off-line coating operations, the coating levels of the machine direction (“MD”) and cross-machine direction (“CD”) yarns are generally not independent.
Unequal coating levels between the MD and CD yarns, normally found in dip coated fabrics, results in an “imbalanced coating weight distribution ratio” in which more coating is deposited on the low tension CD yarns than on the relatively higher tension MD yarns. This “imbalance” often leads to undesirable properties of reinforcements especially those which have been treated or coated for corrosion or fire resistance. In corrosive environments, such as cement-based matrices, heavier coating in the CD implies lower, possible inadequate coating protection on the MD. Both quantity and quality of coating in the MD suffers. The tensioned, twisted MD bundle does not allow coating to penetrate within the bundle. As a result substantial pockets of air remain in the MD bundle. The poor quantity and quality of coating on the MD strands leads to poor corrosion protection of the MD strands relative to that of the CD strands, especially in an alkali environment like cement.
A balanced coating weight distribution is desirable. It is easy to achieve in the case of single-end-coated (SEC) fabrics as each strand is independently and explicitly coated with a given level of coating. The coated strands are then combined into a fabric with the ratio of coating weight (DPUcd/DPUmd) being established simply by selection of yarns containing the desired coating weights-often selected to be the same in MD and CD.
Current glass scrim reinforcements typically include a coating of PVC plastisol, a blend of PVC particles dispersed in plasticizer (usually phthalate based). By necessity, such coatings contain heat stabilizers and varsol (or other paraffin oil based solvent) to control viscosity. Despite the presence of the heat stabilizer, it is not advisable to dry PVC at too high a temperature or it will de-polymerize. The phthalate plasticizer has also come under increased scrutiny for its VOC emissions. The solvent used to control viscosity also tends to evaporate during drawings and yields voids in the coating, leading to decreased alkali resistance in certain locations such that cement, being alkaline, will dissolve uncoated glass fiber surfaces.
The manufacturing plant will also need to burn off the solvent with an oxidizer. Since the plasticizer has some affinity for the PVC particles dispersed in it, it becomes more viscous over time as the plasticizer migrates into the PVC. This problem is exacerbated in the summer months when the manufacturing plant is much warmer. A plant chemist is required to adjust viscosity to compensate for evaporation using additional solvent. This can lead to some variability in coating quality and thickness. Often, batches of plastisol are lost when the viscosity goes too high. In the dip process, where an entire fabric is immersed in the plastisol while it is on the tenter frame, there is much more tension in the yarns in the machine direction than the yarns in the cross machine direction, so much more plastisol is picked in the cross machine direction yarns during coating, resulting in an imbalanced coating. This necessitates using an unnecessarily large amount of plastisol on the scrim so that the machine direction yarns are properly coated to get the same performance as the cross machine direction yarns.
Accordingly, there remains a need for woven, knit or mesh-type non-woven (“scrim”) fabrics which have a uniform coating of an alkali-resistant material which is easily applied, and which generates less VOCs, but which is also drapable and strong, as well as methods for producing such coating on yarn for improving manufactured properties of cement boards, for example, and for protecting these fabrics in environments which require corrosion and flame resistance, for example.